Papermaking screen

ABSTRACT

A papermaking screen includes at least one individual fabric for the paper side and at least one individual fabric for the machine side. Each of those fabrics is provided with a set of weft yarns ( 4, 6 ) and warp yarns ( 1, 5 ). At least one part of the superimposed individual fabrics is interconnected via binder yarns ( 3 ). The two fabric layers (paper side and machine side) are connected by binder yarns that are fully integrated into the fabric structure of the paper side and are able to support the resulting binding point due to each binder yarn ( 3 ) engaging with warp yarns ( 1 ) of the individual fabric from above at defined points on the paper side, while at least one weft yarn ( 2 ) of the individual fabric engages with the warp yarns ( 1 ) from below on the opposite side so as to lean against it such that the binder yarns remain on one plane along with the weft and the remaining warp yarns. This arrangement results in a high-strength papermaking screen which has very good dewatering power and a regular structure.

FIELD OF THE INVENTION

The present invention relates to a papermaking screen having at leastone single fabric for the paper side and at least one single fabric forthe machine side. Each fabric has a set of weft yarns and warp yarns. Atleast a part of the stacked individual fabrics are connected to eachother by binder yarns.

BACKGROUND OF THE INVENTION

Use is increasingly made today in the papermaking industry ofhigh-performance papermaking machines running at speeds of up to 2000m/min and with operating widths exceeding 10 m. As a very general rule,the sheet forming unit is configured as a twin-wire former, in manyinstances as a gap former. In typical machines, the sheet formingprocess takes place immediately in a relatively short dewatering zonebetween two papermaking screens. The time for sheet formation is reducedto a few milliseconds because of this short distance and the high outputrate. The solid content or dry content of the fiber suspension must beraised from about 1 percent to about 20 percent over this interval. Thisoperation requires the papermaking screens to possess high dewateringcapability without leaving marks in the paper and with providing highfiber support.

Another important point is the cross-directional stability of screentension, which is decisive in determination of the thickness andmoisture content profile of the sheet. Very high and preciserequirements have been set for this stability in the case of modernmachines operating with great sheet widths. Consequently, use isincreasingly being made in the sheet forming zone of forming stripsmounted on the machine sides of the screens and pressed against them toimprove the forming. This use results in rapidly changing deflection ofthe fabric of the screens in the longitudinal direction.

There are basically two different approaches in the state of the art tosolve the problem of meeting these requirements, and in particular toeffect binding together of the individual fabrics of the paper side andthe machine side. One solution is characterized in that the twoindividual fabric layers are connected to each other together by a weftor transverse yarn. Another solution provides that the connecting iseffected by a longitudinal or warp yarn. However, the known approachesare now unsuitable if it is desired to employ different warp diameterson the machine and paper sides.

If the configuration of the two individual fabrics accordingly isoriented especially toward a fine paper side with small diameters and acoarse machine side with large diameters in order to arrive at highstability values, connecting of the two layers must be effected by aweft, a binding weft in particular. The state of the art offerscorresponding proposals for solution of this problem as well.

Hence the possibility exists of interweaving the two individual fabricsby an additional binding or stitching yarn which belongs neither to thebinding pattern of the upper fabric (paper side) nor to that of thelower fabric (machine side). Such a solution is disclosed, for example,for the papermaking screen of U.S. Pat. No. 5,238,536, which provides alinen binding and a five-shank binding for the lower fabric. There arealso approaches involving additional stitching yarns whichsimultaneously effect connecting of the two fabric layers and inaddition serve as filling threads. Such a solution is disclosed in U.S.Pat. No. 5,518,042, for example.

In such disclosed solutions, the additionally used binder yarns alterthe inherently homogeneous upper side. In a practical application, thisleads to some extent to undesirable marks in the paper. To counter thisresult, the binder yarns are made to be increasingly thinner, but thishas the disadvantage that the service life of the connection ofindividual fabric layers is correspondingly shortened. In addition,practical application has shown that “looping through” of the bindingweft yarns may occur, this resulting in separation of the individuallayers and rendering the fabric unusable.

In another disclosed solution, complete upper wefts are replaced bypairs of binding structural yarns. Depending on the type of fabric, theratio of true upper wefts made by weft or warp yarns to the binding weftpairs varies. For example, PCT publications WO 99/06630 and WO 99/06632disclose fabrics in which the upper fabric is made as a type of linenbinding by combining two binding weft yarns. The lower fabric in turn isin these disclosed solutions in the form of a five-shank binding.

Despite the good interconnection of the two individual fabrics, thedisclosed solutions present the essential disadvantage that the upperwarp of the paper side is not supported at the points of intersection ofthe binding wefts. If the course of a “complete” upper weft in thesesolutions is considered, it is seen that both yarns are positioned at anelevated level as a result of alternate binding of upper weft and upperwarp, with the result that both the warp and weft bends are positionedin one plane. As a result of use of the binding pairs, this support isnow absent from all intersections and all yarns absorb the main forcesalong their respective longitudinal axis which at the intersections isoriented in the direction of the interior of the fabric. Thisdisadvantage of absence of support arises especially when upper weft andbinding pair are introduced in alternating sequence, so that, forexample, a complete upper weft follows a binding pair which is thenfollowed by an upper weft. To produce the preferably disclosed linenbinding, the following upper weft must extend above the warp yarn whichwas previously positioned above the intersection and as a result isadditionally pulled into the interior of the fabric. The result is thateither every other upper warp yarn is positioned deeper in the fabric ornone of the warp yarns may be positioned at the level of the weft yarns.This arrangement leads to uneven progress of the fabric on the paperside, something which may result in undesirable marks in the paper.

SUMMARY OF THE INVENTION

Objects of the present invention are to provide a papermaking screencharacterized by high stiffness values, in particular a high degree ofcross-directional stability, and affording dewatering output comparableto that of the disclosed solutions and helping prevent formation ofmarks in the paper.

These objects are basically attained by a papermaking screen where therespective binder yarn extends above warp yarns of the individual fabricat specific points on the paper side, below which at least one weft yarnof this individual fabric extends on the opposite side. Connection ofthe two fabric layers (paper side and machine side) is in turn effectedby binder yarns which are then nevertheless fully integrated into thefabric structure of the paper side and in the process support thebinding point by the special type of connection in such a way that thebinder yarns remain on one plane with the wefts and the remaining warpyarns. Application of this binding concept results in production of apapermaking screen having a high degree of stiffness, possessing gooddewatering properties and uniform structure, on the paper side inparticular, so that undesirable marks in the paper are prevented.

As a result of the present invention, the warp yarns are supported frombelow by the associated weft yarns of the individual fabric of the paperside at points at which the warp yarns are pulled into the interior ofthe fabric by the binder yarn. The functional separation of upper andbinding weft also makes it possible to employ for the upper weft (paperside) a material which supports the cross-directional stability of thefabric, such as a polyester material. Both materials are of the sametype in the solutions referred to in the foregoing in use of a bindingweft pair and require optimization with respect to connection of thelayers, use customarily being made of polyamides. Although only onebinder yarn is used in a given plane of the present invention, thenumber of binding points, and accordingly contact between binding weftand upper and/or lower chains of paper side and machine side, are notreduced in comparison to the disclosed solutions.

In one especially preferred embodiment of the papermaking screen of thepresent invention, provision is made such that the diameter of thebinder yarn corresponds to that of the upper weft, resulting in a highdegree of stiffness of binding of the fabric layers.

Other objects, advantages and salient features of the present inventionwill become apparent from the following detailed description, which,taken in conjunction with the annexed drawings, discloses preferredembodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings which form a part of this disclosure:

FIGS. 1 and 2 are diagrammatic, end elevational views in section of twoconventional papermaking screens with binding solutions;

FIG. 3 is a top plan view of a portion of the upper or paper side of apapermaking screen according to a first embodiment of the presentinvention;

FIG. 4 is an end elevational view in section taken along line A-A ofFIG. 3;

FIG. 5 is an end elevational view in section taken along line B-B inFIG. 3;

FIG. 6 is a top plan view of the upper or paper side of a papermakingscreen according to a second embodiment of the present invention;

FIG. 7 is an end elevational view in section taken along line C-C ofFIG. 6;

FIG. 8 is an end elevational view in section taken along line D-D ofFIG. 6; and

FIG. 9 is a top view of the upper or paper side of a papermaking screenaccording to a third embodiment of the present invention, correspondingto the first embodiment, but executed with an alternating weft sequenceof the upper weft and binding weft.

DETAILED DESCRIPTION OF THE INVENTION

The following numeral identifications are employed in all theillustrations:

-   1 upper warp-   2 upper weft (with binding weft)-   3′3 binding weft-   4 upper weft-   5 lower warp-   6 lower weft-   7 extension above-   8 extension below-   9 extension above through lower weft 6

In FIG. 1, the conventional papermaking screen, as viewed in FIG. 1, hastwo individual fabrics, the upper individual fabric or upper fabricforming the paper side and the individual fabric positioned below itrepresenting the bottom side or lower fabric. The upper individualfabric is formed of a set of weft yarns 2 as upper weft yarns and warpyarns 1 as upper warp yarns. The machine side located underneath alsohas a set of weft yarns 6 as lower weft yarns and warp yarns 5 as lowerwarp yarns. The disclosed screen has a linen bond as the binding typefor the paper side. The lower fabric is configured as a five-shankfabric with respect to a repeat. As FIG. 1 shows, the two individualfabrics are connected to each other by a binding weft yarn 3. Aplurality of such binding weft yarns (not shown) are positioned insequence so as to extend into the plane of the drawing and out of it.Thus, the connection of the individual fabric layers necessary for thepapermaking screen is effected. In these disclosed screens, the binderyarns 3 are used in the direction of the fabric in advance of and beyondthe upper weft yarns 2 to effect connection of the individual fabriclayers. Such binder yarn alters the actually very homogenous upper sideof the paper side of the screen disadvantageously such that in practicalapplication undesired marks may appear in the paper. For the bindingweft yarns 3 accordingly to be disturbed as little as possible, they aremade to be increasingly thinner, so that, when the disclosed papermakingscreens are used, separation of the individual fabric layers may occurand accordingly failure of the screen as such.

In the other disclosed screen shown in FIG. 2, two binding weft yarns 3and 3′ accordingly are used. The diameter of those weft yarns isselected to be greater than the diameter of the binding weft yarn 3shown in FIG. 1. As a result of use of the two binding weft yarns 3 and3′ no complete upper weft is present then in these places any longer.The linen binding of the upper side rather is affected through thecombination of the two binder yarns 3, 3′. In this instance as well,only a part of the papermaking screen is shown in section. A pluralityof binder yarns 3 and 3′ are present in sequence in different possibledrawing planes. A significant disadvantage is to be seen in this screenin that the upper warp yarns 1 are not supported at the intersections ofbinder yarns 3 and 3′. In the case of this solution, irregularitiesoccur and accordingly also marks in the paper with respect to the paperside of the screen, since, in order for the linen binding to beeffected, the next upper weft must extend above the warp yarn, whichpreviously was positioned above the intersection and consequently inaddition is pulled into the interior of the fabric. Either every otherupper warp yarn is positioned deeper in the fabric or none of the warpyarns are at the level of the weft yarns, resulting in the disadvantagesdescribed.

The papermaking screen of the present invention will now be describedbelow. For the sake of simplicity and greater ease of understanding, thesame reference numerals are used for the following illustrations ofsolutions, as well as for the conventional screens described in theforegoing.

The first exemplary embodiment of a papermaking screen as shown in FIGS.3, 4, and 5 is provided on the paper side with linen binding. On thelower side or machine side, it is configured as a five-shank bond. FIG.3 presents a top view of a section of the upper or paper side of thepapermaking screen of the present invention, and section A-A shown inFIG. 4 is presented as a view of the upper weft without binding weft,while section B-B in FIG. 5 is a view of the upper weft with bindingweft as shown in FIG. 3.

FIG. 5 in particular shows how connection of the two individual fabricsfor paper and machine side is affected by binder yarns 3. By way ofexample, the progress of such a binder yarn 3 is shown in the form of asection in FIG. 5. This binder yarn is fully integrated into the fabricstructure on the paper side, in that on the paper side the respectivebinder yarn 3 extends above the associated warp yarns 1 of theindividual fabric at specific points. At least one weft yarn 2 of thisindividual fabric extends below these warp yarns 1 on the side oppositethe latter. This extension or point above or below is indicated in FIG.5 by reference numerals 7 and 8. As a result of such arrangement, inwhich the associated binder yarn 3 extends above a warp yarn 1 and theassociated upper weft yarn 2 extends below it, the binding point issupported from the direction of the opposite side, this ensuring thatthis binding point will remain on one plane with the other weft and warpyarns 4 and 1. Consequently, the upper weft yarn 2 also extendsuniformly at the point at which the fabric binding has been effectedwithout being bound into the lower fabric. Only at points at which thebinding weft 3 extends above the upper warp is a brief exchange of upperweft 2 and binding weft 3 carried out. As a result, the warp yarns 1positioned in between are supported from below by the upper weft yarn 2,as seen in FIG. 5, at the points at which these yarns 1 are pulledthrough the binder yarn 3 into the interior of the fabric. Acontribution also is made to support by the warp yarns 5, of greaterdiameter, of the lower weft 6, especially the lower warp yarn 5, whichis positioned vertically below the warp yarn 1 above and below whichbinder yarns extend.

Also in FIG. 5, the respective binding weft yarn 3 defines at the pointof extension above 7 of the associated warp yarn 1 an angle with thelatter. The measurement of that angle equals that of the weft yarn 2extending below at this point. These angular measurements range from 90°to 130° in these areas, as a function of the configuration of thepapermaking screen. As a result of these angular measurements, a sort ofroof pane is created, both on the side of extension above 7 and in theopposite direction at the point of extension below 8. This situation hasa favorable effect with respect to the binding pattern and the overallpattern of forces of the papermaking screen.

The binding of the present invention, configured as a five-shank bindingwith respect to a repeat, provides that the weft yarns 6 extend belowfour warp yarns 5 and above a following warp yarn 5. The respectivebinding weft yarn 3 rises obliquely from the lower fabric to the upperfabric at the point of this extension above 9. The respective bindingweft yarn 3 is essentially of the same diameter as that of therespective weft yarn 2 of the individual fabric on the paper side. Inaddition, the warp yarns 5 and weft yarns 6 of the lower fabric, thatis, on the machine side, are of a diameter larger than that of theassociated yarn systems on the upper or paper side of the papermakingscreen. With respect to the upper or paper side of the screen, therespective extension above 7 of the respective binder yarn 3 isseparated in sequence from a weft yarn 2 by three warp yarns 1positioned between them. At the point of the center, warp yarn 1 of thisgroup of three the binding weft yarn 3 executes extension below yarn 6,a short distance in advance of the extension above 9, with a warp yarn 5positioned underneath. As a result of the functional separation of upperweft yarns 2 of the upper fabric and binding weft yarns 3, these twosets of yarns may be formed of different materials. Preferably, theupper weft yarns 2 are formed of a polyester material and the bindingweft yarn 3 are formed of a polyamide material, for the purpose ofincreasing the cross-directional stability of the screen.

The upper weft yarn 4 shown in FIG. 4 corresponds in configuration tothe upper weft yarn 2 with binding weft yarn 3 positioned upstream fromit as illustrated. The difference in numbering was selected exclusivelyfor the sake of better understanding of the top view of the fabricpattern shown in FIG. 3.

The modified embodiment shown in FIGS. 6, 7, and 8, this embodimentcorresponds largely to the first embodiment initially described, exceptthat a four-shank binding is used here for the lower fabric or lowerside (machine side) in place of a five-shank lower side. In therespective four-shank binding shown in FIG. 8, the warp yarn 1 above,which the binder yarn 3 extends and below which upper weft yarn 2extends, is in turn supported by a warp yarn 5 positioned below it ofthe lower fabric. The lower weft yarn 6 extends above the lower warpyarn 5 at the point of support. The binder yarn 3 is then tied in forthe lower fabric in the area of three consecutive lower warp yarns 5.The binder yarn 3 extends above the central lower weft yarn 5 of a groupof three and above two adjacent lower warp yarns 5. The roof-likeconfiguration in the area of extension above 7 for the upper warp yarn 1has corresponding to it in parallel a configuration in the form ofextension 9 of the lower weft yarn 6 above the supporting lower warpyarn 5.

In the embodiment illustrated in FIG. 9, the sequence of upper weft 2with binder yarn 3 is changed, with the result that all floats of thewarp yarns 1 are of the same length L on the upper side despite thearrangement of the binding points so as to be easily displaced towardeach other. This arrangement makes certain that the warp bends arepositioned in one plane on the upper side, in both the transverse andthe longitudinal directions, and has a favorable effect in view ofslight possible marking of the paper and of the high degree of stiffnessof the screen.

A high degree of stability is achieved with the papermaking screensolution of the present invention. The screen is characterized by verygood dewatering output and its production is cost-effective.

While various embodiments have been chosen to illustrate the invention,it will be understood by those skilled in the art that various changesand modifications can be made therein without departing from the scopeof the invention as defined in the appended claims.

1. A papermaking screen, comprising: a paper side individual fabrichaving a set of upper weft yarns and upper warp yarns; a machine sideindividual fabric having a set of lower weft yarns and lower warp yarnsand being positioned below said paper side individual fabric; and binderweft yarns connecting said fabrics and extending above said upper warpyarns at first points where adjoining ones of said upper weft yarnsextend below said upper warp yarns, said first points of said binderweft yarns being separated by groups of three of said upper warp yarns,said binder weft yarns extending below said lower warp yarns atlocations of central ones of said groups of three of said upper warpyarns.
 2. A papermaking screen according to claim 1 wherein only a firsttype of said binding connects said fabrics.
 3. A papermaking screenaccording to claim 1 wherein said paper side individual fabric isconfigured as linen binding; and at said first points, said binding weftyarns form angles equal to angles formed by said adjoining ones of saidupper weft yarns where said upper weft yarns extend below said upperwarp yarns.
 4. A papermaking screen according to claim 1 wherein saidmachine side individual fabric comprises a multiple shank binding; oneof sets of three and four of said lower warp yarns extend above saidlower weft yarns and sets of one of said lower warp yarns extend belowsaid lower weft yarn at second points; and said binding weft yarnsextend from said machine side individual fabric to said paper sideindividual fabric at one of spaced from said second points and at saidsecond points.
 5. A papermaking screen according to claim 4 wherein setsof three of said lower warp yarns extend above said lower weft yarns. 6.A papermaking screen according to claim 4 wherein sets of four of saidlower warp yarns extend above said lower weft yarns.
 7. A papermakingscreen according to claim 1 wherein said binding weft yarns havediameters essentially equal to diameters of said upper weft yarns.
 8. Apapermaking screen according to claim 1 wherein said binding weft yarnsare supported by upper wet yarns at said first points, and extendbetween said upper weft yarns and said lower weft yarns.
 9. Apapermaking screen comprising: a paper side individual fabric having aset of upper weft yarns formed of polyester material and upper warpyarns; a machine side individual fabric having a set of lower weft yarnsand lower warp yarns and being positioned below said paper sideindividual fabric; and binder weft yarns formed of a polyamide materialconnecting said fabrics and extending above said upper warp yarns atfirst points where adjoining ones of said upper weft yarns extend belowsaid upper warp yarns, said upper weft yarns and said binding weft yarnsbeing separate and formed of different materials; wherebycross-directional stability is increased.
 10. A papermaking screenaccording to claim 9 wherein only a first type of said binding weftyarns connects said fabrics.
 11. A papermaking screen according to claim9 wherein said paper side individual fabric is configured as linenbinding; and at said first points, said binding weft yarns form anglesequal to angles formed by said adjoining ones of said upper welt yarnswhere said upper weft yarns extend below said upper warp yarns.
 12. Apapermaking screen according to claim 9 wherein said machine sideindividual fabric comprises a multiple shank binding; one of sets ofthree and four of said lower warp yarns extend above said lower weftyarns and sets of one of said lower warp yarns extend below said lowerweft yarn at second points; and said binding weft yarns extend from saidmachine side individual fabric to said paper side individual fabric atone of spaced from said second points and at said second points.
 13. Apapermaking screen according to claim 12 wherein sets of three of saidlower warp yarns extend above said lower weft yarns.
 14. A papermakingscreen according to claim 12 wherein sets of four of said lower warpyarns extend above said lower weft yarns.
 15. A papermaking screenaccording to claim 9 wherein said binding weft yarns have diametersessentially equal to diameters of said upper weft yarns.
 16. Apapermaking screen according to claim 9 wherein said binding weft yarnsare supported by upper weft yarns at said first points, and extendbetween said upper weft yarns and said lower weft yarns.
 17. Apapermaking screen comprising: a paper side individual fabric having aset of upper weft yarns and upper warp yarns; a machine side individualfabric having a set of lower weft yarns and lower warp yarns and beingpositioned below said paper side individual fabric; binder weft yarnsconnecting said fabrics and extending above said upper warp yarns atfirst points where adjoining ones of said upper weft yarns extend belowsaid upper warp yarns, said upper weft yarns and said binding weft yarnsbeing separate and formed of different materials, said first points ofsaid binder weft yarns being separated by groups of three of said upperwarp yarns, said binder weft yarns extending below said lower warp yarnsat locations of central ones of said groups of three of said upper warpyarns.